1. Field of the Invention
The present invention relates to an electrophotographic printer for printing images on a recording medium such as paper. The present invention also relates to a transfer charger incorporated in such a printer for transferring images onto a recording medium.
2. Description of the Related Art
Electrophotographic printers have been widely used for printing desired images on a recording medium such as continuous paper, cut sheets and so on. FIG. 11 of the accompanying drawings shows the outline of a conventional electrophotographic printer.
As illustrated, the conventional printer includes a photosensitive drum 90, a latent image forming unit 91, a developing unit 92, a transfer charge unit 93, a fixing unit 95, a pair of tractors 94 (only one shown) and so on. The tractors 94 are provided for feeding continuous recording paper P along a predetermined paper transfer path. For this purpose, though not illustrated, use is made of paper feed rollers to be operated together with the tractors 94.
In the conventional printer, electrostatic latent images are formed on the annular surface 90a of the drum 90 by the image forming unit 91. The latent images are developed by the developing unit 92 to produce visible toner images, and then these toner images are transferred onto the paper P by the transfer charge unit 93. Thereafter, the transferred images are fixed to the paper by the fixing unit 95.
The conventional transfer charge unit 93 is provided with only one transfer wire 93a and one static-eliminating wire 93b. The transfer wire 93a allows the passage of electric current (referred to as "transfer current" below) of a predetermined polarity for transferring toner images (formed on the drum 90) onto the paper P. More specifically, the toner images formed on the photosensitive drum 90 are negatively charged, while the transfer wire 93a is rendered positive due to the transfer current. Thus, the toner images on the drum 90 will be attracted toward the transfer charge unit 93 to be transferred onto the paper P. In this process, the paper P is positively charged by the transfer current. In this connection, the static-eliminating wire 93b serves to eliminate the positive charges on the paper P, thereby preventing the paper P from unduly adhering to the drum 90. Consequently, the paper P is properly fed along the predetermined paper transfer path.
Though having the above advantage, the conventional printer may suffer from the following problems.
As stated above, the annular surface 90a of the drum 90 is negatively charged during the printing operation. Thus, when the paper P is brought to the drum 90, part of the paper P will adhere to the annular surface 90a of the drum 90. Specifically, as shown in FIGS. 12 and 13, the adhering amount or manner of the paper P varies depending on the thickness (stiffness) of the paper P. For instance, when the paper P (p') is relatively thin (FIG. 12), a larger area of the paper P' is stuck to the drum 90, and the sticking begins at a position N1. On the other hand, when the paper P (p") is relatively thick (FIG. 13), only a smaller area of the paper p" is stuck to the drum 90, and the sticking begins at a position N2, which is located downstream of the paper transfer path from the above-mentioned position N1.
Keeping the above-described behavior of the paper P in mind, attention is drawn to the transfer charge unit 93 of the conventional printer. As stated above, the conventional transfer charge unit 93 is provided with only a single transfer wire 93a fixed at a position facing the photosensitive drum 90. Even with such an arrangement, it may be possible to perform proper image transfer onto the comparatively thin paper p' shown in FIG. 12, wherein the transfer wire 93a is located immediately downstream from the position N1.
However, when the comparatively thick paper P" is used (FIG. 13), the transfer wire 93a is located upstream from the position N2, which means that the transfer wire 93a faces a portion of the paper p" which has still not come into engagement with the annular surface 90a of the drum 90. In this state, disadvantageously, the toner images on the drum 90 may fail to be properly transferred onto the paper P", so that no good printing results are obtainable.
Further, in the conventional printer, the range over which the transfer current is effective for drawing the toner images toward the recording paper is not variable depending on e.g. the type of paper P. Also, the strength of the transfer current may not be readily adjusted. Clearly, these inconveniences tend to be obstacles to proper transfer of toner images onto the recording paper.